1. Field of the Invention
The present invention relates to compositions and methods for bioremediation of chlorinated biphenyls, and contemplates the use of dechlorinating microorganisms which are effective chloro substituents from biphenyl rings, including for removing ortho-chlorine substituents and doubly flanked chloro substituents.
2. Description of the Related Art
Polychlorinated biphenyls (PCBs) are haloaromatic compounds having exceptional chemical stability. Environmental and toxicological problems caused by the use of PCBs have resulted in restriction of their production under the Toxic Substances Control Act of 1976 and a complete ban of their manufacture by the United States Environmental Protection Agency in 1979. Past disposal practices have resulted in substantial PCB contamination of soils and surface water sediments. As a consequence, in the United States, at least 15% of the PCBs manufactured to date remains in the environment as a highly recalcitrant contaminant. Acute toxicological effects of PCB exposure include chloracne (a skin disease), teratotoxicity, endocrine effects, immunotoxicity, carcinogenicity, and hepatotoxicity (liver damage). The mutagenic and carcinogenic character of PCBs and their suspected role in the reproductive failure of wildlife species are issues of great concern. Further, these compounds bioaccumulate and biomagnify in the fatty tissue of animals in the food web, such as fish, which can affect the human population as a result of food consumption. In sum, the toxicological findings on PCBs and their propensity for bioaccumulation raise concern for the well-being of both humans and wildlife.
Historically, harbor regions have been heavily impacted by the accumulation of polychlorinated biphenyls due to their use in and inadvertent release from naval and industrial applications. Due to their hydrophobic character, PCBs strongly associate with organic carbon, clays and silt that settle into the anaerobic regions of marine sediments.
In aerobic environments, PCBs undergo microbial degradation with oxygen addition at the 2,3 positions by a dioxygenase and subsequent dehydration to catechol followed by ring cleavage. Although lesser chlorinated PCBs ranging from mono- to hexa-chlorinated congeners can be degraded aerobically, extensively chlorinated congeners (e.g., tetrasubstituted) such as those prominent in Aroclor 1260, a formerly commonly used PCB material, are not transformed under aerobic conditions. In this respect, most aerobic degradative activity is restricted to congeners with less than 4 to 6 chlorines, depending on the positions of the chloro substituents on the rings. This is a small region of the structural spectrum of PCBs, since there are 209 congeners (isomers and homologs) of PCBs. The substantial variety of congeners is evidence from the structure of the biphenyl molecule (see FIG. 20 herein). Commercial mixtures of PCBs formerly marketed in the United States under the Aroclor trademark typically contained more than 50 of such congeners. The extent of chlorination of the PCBs varies with the specific commercial mixture. For example, Aroclor 1242 is dominated by tri- and tetrachlorobiphenyls, the aforementioned Aroclor 1260 is dominated by penta-, hexa- and heptachlorobiphenyls, and Aroclor 1268 is dominated by hepta-, octa- and nonachlorobiphenyls. Even less-chlorinated Aroclors contain significant levels of congeners with 5 or more chlorine substituents. For this reason, even a consortium of aerobic bacteria (a consortium being a population of bacteria containing different strains with different congener (degradative) & specificity) cannot remove Aroclor PCB compositions from the environment.
Anaerobic dechlorination of PCBs is a critical step in the biodegradation of these anthropogenic compounds in anaerobic sediments. However, a general knowledge of the microorganisms responsible for these reactions has eluded prior isolation, identification and characterization efforts.